1. Field of the Invention
The present invention relates to a T-shaped film extrusion die for use in forming films or sheets consisting of a synthetic resin, such as polyethylene, polypropylene and ethylene vinyl acetate, and more particularly to a T-shaped film extrusion die suitable, especially, for forming films or sheets out of a synthetic resin, such as polypropylene and ethylene vinyl acetate, without producing undesirable edge beads (unduly thick portions of a film produced at both edges thereof) and local thicker portions (non-uniform in thickness) which are encountered in a conventional film of such a resin.
2. Prior Art
The prior methods for forming polyethylene or polypropylene into films include an inflation method and a method using a T-shaped film extrusion die. Out of these methods, the latter has been utilized to produce a fertilizer bag, a moisture-proof bag or the like by extrusion forming polyethylene or the like into a film, roll pressing the thus formed film while it is hot on a metal foil, cellophane paper, kraft paper or the like to make a laminate thereof. This prior method has been widely used for the production of a wrapping material for medicine and food and of various kinds of resin sheets as well.
There have been proposed various experimental methods for producing suitable films efficiently by using such an T-shaped film extrusion die which has been later improved. These methods are illustrated in, for example, Japanese Utility Model Registration Nos. 1726281, 1603083 and 1777587 and Japanese Patents Nos. 1568629 and 1631933. All of these utility models and patents are owned by the applicant of the present application.
In cases where the conventional T-shaped film extrusion dies described in the above publications are used, films having high transparency are generally obtained, and, when a particular resin such as polyethylene is used, substantially satisfactory products which are uniform in thickness and free from edge beads are obtained.
However, the synthetic resins used in a film forming method using a T-shaped film extrusion die have recently been greatly diversified, and, besides high-, intermediate- and low-density polyethylenes (PE) which have been conventionally used, various kinds of resins, such as polypropylene (PP), ethylene vinyl acetate (EVA), ionomer, LLDP, EVOH and EAA have come to be used. Further, since the properties of a resin will vary depending on a resin producing method such as an autoclave method or a tubular method, it can be said in this respect as well that the synthetic resins used in a method using a T-shaped film extrusion die widely vary in properties.
Therefore, in a method using the conventional T-shaped film extrusion die, the following problems still arise, i.e., edge beads occur at both edges of a film or the thickness of the film is liable to become uneven depending on the kind of a resin used. Such problems occur remarkably especially when polypropylene or ethylene vinyl acetate is used.
FIGS. 5-7 show an example of a conventional T-shaped film extrusion die disclosed in Japanese Patent No. 1631933. FIG. 5 is a front elevation of this conventional T-shaped film extrusion die, FIG. 6 an enlarged sectional front elevation of the portion of this die which is in the vicinity of a side plate, and FIG. 7 a sectional view of the same T-shaped die taken along the line Y--Y in FIG. 5.
Referring to FIGS. 5-7, the T-shaped film extrusion die 1' comprises a manifold 2', a land 3', lips 4', inner deckles 5' and inner deckle rods 6'. The inner deckles 5' are supported by support rods 12' and positioned so as to extend from the upper end of the manifold 2' to the lower end of the land 3'. The inner deckle rods 6' are positioned at the lowermost portion of the land 3' so as to be spaced from the inner deckles 5' via a predetermined level of clearance. Each of the inner deckles 5' is divided into an upper inner deckle portion 5a' and a lower inner deckle portion 5b'.
Referring to FIG. 5, the arrows represent the flow of a resin. A molten resin extruded from an extruder A' flows into the land 3' through the manifold 2', and the width of the flow of the resin is restricted by the inner deckles 5' and strictly set to a final predetermined level by the inner deckle rods 6' simultaneously with the thickness of the resin being controlled by the lips 4', so as to obtain a desirable film.
In the conventional T-shaped film extrusion die, the temperature of the die is generally maintained at a temperature about 5.degree.-10.degree. C. higher than that of a molten resin (temperature of the resin at the inlet port of the T-shaped die) by means of external heaters (not shown) provided on the outer portions of the T-shaped extrusion die body so that the heaters are spaced at intervals of about 180 mm. However, the temperature of the resin is not necessarily uniform in the T-shaped die due to differences in distance between portions of the resin and the heaters and also due to the generation of heat by the resin. In the conventional T-shaped film extrusion die, a molten resin tends to flow faster at the central portion of a die body as shown by arrows in FIG. 5 than that at both end portions thereof. Therefore, depending on the kind of a resin used in the conventional T-shaped die, edge beads and/or non-uniform thickness will occur in the resultant film due to the uneven temperature and flow of the resin. Especially, when polypropylene or ethylene vinyl acetate is used, edge beads and/or non-uniform thickness of the resulting film will always occur.
Moreover, in the conventional T-shaped film extrusion die, the resin residing in a space (hatched portion in FIG. 6) between the inner deckles 5' and side plate 16' leaks from a clearance between the inner deckle rods 6' and lips 4' to be deposited on the film extruded from the die. The frequency of occurrence of leakage of the resin is specially high when a film having a small width is produced. The leakage of the resin occurs at a frequency of as high as not less than once per 3-5 minutes in some cases, so as to cause a decrease in the quality and yield of final products. Since the leaked resin turns into an oxidized resin and is deposited on the circumferential portions of the lips 4', the movement of the inner deckle rods 6' is obstructed and, further, it is necessary to remove the oxidized resin when the width of a film to be obtained is desired to be changed.